I worked at a company that went through this. Honestly, it changed the entire mood of the company and working there. We went from thousands of messages per day to something like 10 (of those channels I was part of, at least). People just hated it, and only used it if they really, really needed to. No more bouncing of ideas around, no more ribbing, just the desperate 'who do I talk to about accomplishing X, anyone know?'
A business owner might conclude 'ah, less time jawing, more time working', but hardly the case. In fact, I think that was a big factor in what ultimately killed the company off a couple years later - through both people literally quitting over it, and a complete breakdown in communication.
I'm no UX expert but I'm going to claim it's because of the UX that Teams doesn't work for so many people, and I'm left wondering why Teams hasn't had a UX overhaul yet.
The other competitor to Slack is Discord, and if you remove the playful "gamer" elements I think it'd be a lot less jarring to people used to Slack, because they follow a lot of similar UX and design patterns.
At one point Discord tried to rebrand into something a bit more serious but it didn't work, but I think they should try again; create a Discord Pro or something like that, get the certifications, add SSO support, etc.
A business owner might conclude 'ah, less time jawing, more time working', but hardly the case. In fact, I think that was a big factor in what ultimately killed the company off a couple years later - through both people literally quitting over it, and a complete breakdown in communication.